The present invention relates to high temperature gas seals, particularly for use in the cells of a solid oxide fuel cell stack.
A planar solid oxide fuel cell (SOFC) stack has three primary constituents, a ceramic electrochemical cell membrane, metallic interconnects, and an arrangement of seals. To perform the function of converting chemical energy into electrical energy, a SOFC membrane must have one electrochemical face exposed to an oxidant gas, and the other exposed to a fuel gas, all at an operating temperature at or above 600° C. A metallic interconnect provides fuel and oxidant gas distribution to the cells by means of separate plenums, and when arranged between cells in a fuel stack arrangement, also transfers electrical current from one cell to another. The seals required between a cell and an interconnect in a SOFC stack must provide adequate resistance to gas permeation to contain the reactants within the gas distribution plenum, as well as provide adequate electrical isolation between the cell and interconnect. The seal should preferably resist significant degradation over time, and it should preferably be capable of being thermally cycled.
Conventional sealing methods all have disadvantages for use in planar SOFC stacks. Most prior art seals are glass which has been crystallized between the two members to be sealed, forming a brittle gas tight seal. The difficulties with glass seals arises from the need to thermally cycle the stack from room temperature to operating temperatures of 700-900° C. The various stack components tend not to have their coefficients of thermal expansion perfectly matched, thus stresses arise during thermal cycling of the cell. Even if the coefficients of thermal expansion are matched, the rates of thermal conductivities within a stack are typically not matched, resulting in non-uniform thermal expansion. As glass is inherently brittle, it cracks and fails under thermal cycling conditions. The brittleness of glass also makes glass seals subject to failure as a result of jarring shocks or vibrations. This is often the case in cells used in motor vehicles. A further disadvantage of glass seals is chemical incompatibity with electrocatalytic cells, which leads to power degradation under operation. A SOFC is particularly sensitive to alkali elements contained in many glass seals which can detrimentally affect the SOFC catalyst.
Other prior art seals have been made of mica, and while being able to withstand the high temperature, they are typically unable to provide an adequate seal to keep the fuel cell input gases separated. In addition, it has been found that the mica may leach minerals into the cell and poison the catalyst. Further problems have been found with the natural variance in thickness of mica sheets and the relative non-compressibility of the mica. Both of these factors prevent an effective seal from forming.
Therefore, there is a need in the art for a seal suitable for use in a SOFC which mitigates the difficulties found in the prior art.